He points out several situations where HIV prevalence could be expected to rise, for “good” reasons. Obviously, the longer people survive with HIV, the more people there will be living with HIV at any one time. So yes, increased access to life-prolonging medicine can be expected to result in a rise in HIV prevalence. Does that mean lower prevalence is a “bad” thing? Not necessarily.

The cries of victory from Zimbabwe are based on prevalence rates among pregnant women aged 15-24. HIV prevalence among women in this age group has fallen by more than a quarter in urban areas from 2000 to 2006. Rates seem to have drifted downwards in rural areas too, but not significantly. Could this be, as Tim posits, because more people are dying more quickly? It is possible, of course, but by restricting the analysis to women under 25 the chances of that are limited. Even in rural areas with exceptionally limited health services, people with HIV live a median of between nine and ten years. You’d have to have a truly massive deterioration in general health to see much of an impact of reduced survival in people under 25.

Tim speculates that lower prevalence may be the result of outmigration. But this only works if there is a selective outmigration of HIV-infected people (and in this case a selective outmigration of HIV-infected women aged 15-24, which seems unlikely).

Could prevalence be falling because there are fewer new infections? In other words because HIV negative people are actually less likely than before to be having unprotected sex with someone who is already infected, and who is highly infectious? Possibly. People could be having no sex at all, or no unprotected sex at all, but that wouldn’t account for any changes we see among pregnant women, because they have by definition had unprotected sex.

Could young women be less likely to be having sex with someone infected? Yes, for example if young girls drop their sugar daddys in favour of sex with boys their own age or younger, since those boys are far less likely to be infected than the older men who can provide nice meals, visits to nightclub and units for the cellphone, as well as HIV infection. Could new infections be falling because young women are less likely to be having sex with someone infectious? Yes, if access to antiretorvirals is expanding and adherence is good, because good treatment regimes lower viral load, and that lowers infectivity in the positive partner.

If new infections are falling because uninfected people are less likely to be having sex with infectious people, it still begs the question: is that the result of prevention programmes, or of other factors such as a “personal witness” effect, as Tim suggests? I think the question is a somewhat spurious — it’s just not possible to disentangle the effects. But lets look for a moment at “personal witness”.

Do people change their sexual behaviour if they see a lot of people around them dying of AIDS, which they know is the result of a sexually transmitted virus? It is very plausible. It seems to have happened among gay men in rich countries, and among men and women in Uganda. Do people change their sexual behaviour if they see a lot of people around them dying, but believe the deaths are the result of poverty, witchcraft, post-colonial malevolence or any of the other things that have been hinted at as causes of AIDS, things that allow people not to think about sex? That seems less plausible. I don’t know what proportion of AIDS-related deaths in Zimbabwe have been recognised as being the result of people’s sexual choices and behaviours, but if the proportion is small (as it is in neighbouring South Africa, for example), the witness effect is unlikely to be large.

My own suspicion is that lower prevalence in Zimbabwe is indeed the result of the country’s economic implosion. Not because of outmigration or reduced survival, but because men can no longer afford to support several women, and older men can no longer afford to buy the things that younger women want in exchange for sexual favours. I’m not the only one who thinks this. Craig Timberg wrote a nice story about it in the Washington Post some months ago.